1. Field of the Invention
The present invention relates to spectrum analyzers. More specifically, the present invention relates to a novel and improved optical RF spectrum analyzer subsystem and system, utilizing a plurality of small, lightweight optical RF bandpass filters each for extracting an optical signal with a particular modulation frequency bandwidth from an optical signal modulated by broadband RF signal, for analyzing the frequency content of the RF signal.
2. Background Art
Future tactical data links, electronic intelligence collection systems and high resolution radar receivers demand high performance, large time-bandwidth product devices for signal processing. Optical fiber and integrated optics technologies promise to provide versatile and effective signal processing techniques with bandwidths and time-bandwidth products exceeding those of any other technology currently envisioned. Other potential benefits include low power requirements; reduced size, weight, cost, complexity; and reduced sensitivity to electromagnetic interference, electromagnetic pulse and nuclear radiation.
Electronic support measures (ESM) intercept receivers are typically required to detect and identify a large number of simultaneous transmissions within a given frequency band. One category of interest within ESM is the ELINT (electronic intelligence) category which can be defined as the "collection of processing of foreign noncommunications radiations".
The frequency range requirements for ELINT receivers may be in the range starting at 0.25 GHz and reaching up to and beyond 32 GHz with a resolution requirement as small as 1 MRz in certain cases. Transmitters of particular interest within the ELINT category are predominantly radars. The parameters that can potentially be used to identify a specific radar include the signal pulse center frequency, bandwidth/spectral signature, pulse width, pulse amplitude, pulse repetition rate, direction of arrival, pulse stagger, and scan modulation.
One of the most effective types of ELINT receivers is the channelized superheterodyne receiver, which typically consists of a large number of contiguous IF filters arranged in a parallel configuration. These receivers have high intercept probability and sensitivity, wide dynamic range, good complex signal capability and excellent preservation of the signal waveform. However, the major drawback of the typical channelized superheterodyne receiver is that it is expensive, large and heavy.
One type of ELINT currently being used is the direct detection receiver which is characterized by a relatively simple receiver tuned to a give band of frequencies and usually a broadband of frequencies. Multiple channel versions have also been developed. These type of receivers are characterized by low sensitivity (due to broad bandwidth); problems with signal sorting in a high signal density environment; and high detection probability with strong signals.
Another type of ELINT receiver is the instantaneous-frequency-measurement (IFM) receiver. This type of receiver is a direct detection receiver containing frequency discriminators which feed a display thereby directly showing signal amplitude and frequency. These receivers are typically undesirable because strong signals will suppress weaker signals which overlap in time, and the signals of nearly equal power, within a few dB, will be averaged in frequency with a single output presented.
In other applications, a digital version of the IFM receiver has been utilized and is well suited to automated receiving systems. However, the digital IFM receiver suffers many of the same shortcomings as the analog IFM receiver.
Another type of ELINT receiver is the superheterodyne receiver which has a fixed IF frequency. The frequency to be detected is mixed with the fixed IF frequency, and electrically filtered for detection of the frequency content. Although this type of receiver has more sensitivity than direct detection or IFM receivers, the local oscillator radiation can leak onto the antenna causing false frequency reading, or nonlinearities in the system can cause spurious responses. Other undesirable features of the superheterodyne receiver is the low probability of detection of short burst transmission and the limitation in receiver sensitivity to a very small portion of the tuning bandwidth at one time.
Another type of ELINT receiver is the previously discussed channelized superheterodyne receiver. This type of receiver is similar to the superheterodyne receiver except that a large number of contigous IF filters are arranged in parallel. Surface acoustic wave (SAW) device filters may be implemented to make this type of receiver possible. This type of receiver has the desirable features of high intercept probability, good complex signal capability, preservation of signal waveform, and high sensitivity. However, this type of receiver also requires high speed signal processing, with an appropriate preprocessor, and is extremely large, complex and costly.
Another type of receiver is the microscan receiver. In this type of receiver, the frequency detector is swept through the receiver frequency range at a time equal to or less than the duration at the shortest pulse in the signal of interest. The sensitivity of this type of receiver is reduced from the superheterodyne receiver because the instantaneous bandwidth must be increased due to the short dwell time on each frequency.
Furthermore, a final type of ELINT receiver is the compressive receiver which essentially performs a chirp transform using surface acoustic wave (SAW) devices, Although the SAW devices are relatively simple, they have a limited upper bandwidth operation range in the GHz range. This type of receiver exhibits a fast serial readout of frequency which places severe demands upon digital circuitry, analog to digital converters, pulse sort processors, and etc. This type of receiver has high data rates and sensitivity along with a high intercept probability. This type of receiver scans much faster than the superheterodyne receiver for the same resolution but requires additional circuitry to extract signal modulation. This type of receiver is also limited to approximately 250 MHz intervals and parallel channels must be used.
The present invention employes a spectrum analyzer subsystem and system with a high probability of detection equivalent to the channelized superheterodyne receiver with a decrease in size and complexity. The approach utilized in the present invention also provides parallel data readout which can be combined into charge transfer devices. This configuration significantly reduces the processing load of the digital circuitry below that required for compressive receivers.
It has previously been impossible to construct an RF spectrum analyzer system based on the channelized concept while utilizing optical techniques due to the inability to filter the modulation on the optical signal. With respect to previous types of optical filters used in optical systems, it is known to construct a segment of optical fiber which is resonant to the optical or carrier frequency by placing highly reflecting mirrors on both ends of the fiber and injecting light of appropriate characteristics into the fiber. A fiber segment so configured may be referred to as a resonant cavity with respect to the carrier frequency. This resonant cavity has been described as being useful for the determination of coupling coefficients so as to enable one to specify and predict the light transmission characteristics of a particular fiber. This test assumes the use of a multimode fiber segment where the coupling coefficients between at least two light propagating modes are simultaneously at resonance within the fiber segment when measured.
However, it has not been suggested prior to applicant's co-pending U.S. patent application Ser. No. 688,271 filed Jan. 2, 1985, now U.S. Pat. No. 4,577,924 issued Mar. 25, 1986, and entitled "Optical Recursive Filter", incorporated by reference herein, which is a continuation of Ser. No. 384,186 filed June 3, 1982, now abandoned, that a multimode optical fiber functioning as a resonant cavity may be employed, with its attendant advantages relating to cost, size, weight and reduced susceptibility to external interference, as an optical RF bandpass filter for filtering RF frequency modulation on a substantially constant carrier frequency optical signal.
It is therefore an object of the present invention to provide a novel and improved RF spectrum analyzer subsystem utilizing noncoherent optical filters in a parallel channelized arrangement.
It is yet another object of the present invention to provide an RF spectrum frequency analyzer subsystem utilizing a laser diode coupled to an external cavity for providing an increased modulation frequency spectrum.